1. Field of the Invention
The present invention relates to an integrated circuit for driving a Liquid Crystal Display (LCD) and more particularly, to a circuit for generating a driving voltage in an LCD driving integrated circuit (referred to as an LCD driver IC).
2. Description of the Related Art
An LCD is a display device used in portable communication devices or home appliances such as handheld computers and personal digital assistants. LCDs display data utilizing the principle that optical transmissivity changes according to the magnitude of voltages applied to both ends of the liquid panel. There are generally two categories of LCDs, namely, STN (Super Twisted Nematic)-LCD and TFT (Thin Film Transistor)-LCD. The methods for driving these LCDs are different.
An LCD driver IC is an IC used to generate a driving voltage required for displaying data on LCD panel. In general, there are electrodes at both ends of the liquid panel, to which a voltage is applied. An electrode at one end of the panel is referred to as the common electrode and an electrode at the other end of the panel is referred to as the segment electrode. A voltage input to the common electrode is referred to as the common voltage and a voltage input to the segment electrode is referred to as the segment voltage.
The LCD driver IC is designed to receive characters or an image to be displayed on an LCD, convert the data of the characters or image into a segment voltage and a common voltage, and apply the converted voltages to the LCD panel.
In general, there are six levels of driving voltages input into a common electrode and a segment electrode of an LCD panel. A circuit for generating a driving voltage generates the six levels of driving voltages. It is important to generate the driving voltages effectively with low power consumption.
FIG. 1 is a block diagram showing a driving voltage generating circuit of a conventional LCD driver IC. The circuit in FIG. 1 is a circuit used for a conventional STN-LCD driver IC. The conventional LCD driving voltage generating circuit 100 includes a DC-DC converter 110, a voltage divider 120 and an oscillator 130. The DC-DC converter 110 is a circuit referred to as a voltage booster and generates a first driving voltage V0 by amplifying a received input voltage VCI by a predetermined amount. The first driving voltage V0 is a high voltage required for driving the LCD panel 140.
Basically, the DC-DC converter 110 boosts a voltage by charging a capacitor with an electric charge via switching and pumping of electric charge. A clock signal CK with a certain period is used as a switching signal required for switching. The clock signal CK is generated in the oscillator 130. The first driving voltage V0 generated by the DC-DC converter 110 is divided by the voltage divider 120 and output as the second through fifth driving voltages V1–V4.
When the LCD panel 140 is driven, power or current consumption in a panel changes according to display patterns, so the level of the first driving voltage V0 also changes. In other words, if the current consumption of the panel is low, the level of the first driving voltage V0 is maintained. However, if the current consumption of the panel is high, the level of the first driving voltage V0 is greatly decreased.
As described above, if the current consumption changes depending on the display patterns and the level of the first driving voltage V0 changes depending on current consumption, the brightness of a display changes depending on the display patterns. It is important to boost the first driving voltage V0 to a certain level because the second through fifth driving voltages V1–V4 are generated based on the first driving voltage V0.
However, if the DC-DC converter 110 uses a fixed frequency clock signal CK, as in the case of using the conventional driving voltage generating circuit 100 shown in FIG. 1, boosting is not performed effectively. Efficiency of voltage booster is influenced by power consumption and boosting efficiency. Namely, it is preferable to use a DC-DC converter which has low power consumption and high boosting efficiency.
It is noted that boosting efficiency, namely a ratio of a target value of the first driving voltage V0 to the first driving voltage V0 is represented as a percentage. Namely, if the target value of the first driving voltage is 10V, and the level of the first driving voltage V0 goes below 8V, the boosting efficiency is 80%. Accordingly, the first driving voltage V0 needs to be maintained at a desired level to increase boosting efficiency regardless of a load of the LCD panel 140.
In general, if the current consumption of the LCD panel 140 is low, it is possible to obtain sufficient boosting efficiency using a clock signal CK having a very low frequency. On the other hand, as the current consumption of the LCD panel 140 increases, the frequency of the clock signal CK needs to be increased to increase boosting efficiency.
However, the conventional driving voltage generating circuit 100 uses a clock signal having a fixed frequency. If current consumption of the LCD panel 140 is low, current is unnecessarily consumed by the DC-DC converter 110. In general, if the frequency of the clock signal CK is high, the current used by the DC-DC converter 110 increases.
On the other hand, if the current consumption of the LCD panel 140 is very high, a clock signal CK having a relatively high frequency is required. However, the conventional driving voltage generating circuit 100 performs voltage boosting with a clock signal having a fixed frequency, dropping the level of the first driving voltage V0. Therefore, display quality is decreased.